1. Field of Invention
This invention relates to the load testing of structures that are lifted or suspended by one or more cables or chains, collectively referred to as “Shipping containers”. The test apparatus applies a load to the lifting points of the containers, known as pad eyes, and simulates the application of loads present in the container when it is lifted or suspended.
2. Background
Shipping containers are designed, manufactured and tested in compliance with one or more specifications published by governmental and private organizations. These specifications require that the container be load tested to a multiple of the rated load of the container. The most popular method used to apply the test load is to fill the container with heavy objects such as concrete, steel, water, etc. This method is time consuming, expensive, and of questionable accuracy.
A second method is to use a hydraulic ram to apply a load to a cable attached to one or more pad eyes, where the reaction of this load is applied to the container near the pad eye being tested. This method applies the load only to the pad eye and not to the entire structure of the container. Therefore, this method does not simulate the actual loads applied the contents of the container when it is in use.
A third method is to use a single telescopic mast to apply a load to cables attached to two or more of the pad eyes. The reaction of this load is applied to the bottom of the structure, more closely simulating the loads applied by the contents of the container when it is in use. This method works well for smaller containers whose length and width are nearly equal, i.e., the container is nearly square. However, it is common for containers to have a large aspect ratio, where the length is several times the width, i.e., the containers are long and narrow.
Containers of large aspect ratios present at least two significant problems to the single telescopic mast method of application of the test load. First, the apparatus must be tall enough to allow the cables to form an angle of not less than 45 degrees with the horizontal. Such a test apparatus must be heavy enough not to fail by buckling when the test loads are applied and, because of the height and weight requirements, the apparatus presents a safety hazard to personnel setting up and operating the apparatus. The second problem with the single telescopic mast method is that the load is applied to a relatively small area at the center of the container unless large, heavy beams are used to distribute the load along the base of the container.
Considerations
A safe and efficient test apparatus for shipping containers should be compact, and easily set-up for testing. It should have a low weight-to-strength ratio, that is, it should be as light-weight as possible while being strong enough to apply and withstand the required loads. Members subject to compressive loads should be as short as possible to increase the resistance of the member to failure or excessive deformation due to buckling. It should reliably apply the test load to the lifting eyes and to the bottom of the container in such a manner so as to accurately simulate the magnitude and direction of an actual load typical to said container. The test load should be evenly distributed among the lifting eyes of the container. The design should provide a means to accurately measure the applied load and minimize the factors that contribute to measurement errors. Such factors include improper set-up resulting in improper alignment of components, improper angle between the cables and a horizontal plane, application of extraneous loads and moments to the load measuring members, and improper distribution of the reaction load to the structure of the container.
Objects and Advantages
Several objects and advantages of the present invention are:
(a) to provide a test apparatus that maximizes safety to personnel while setting up and operating the apparatus, specifically by attaining a low weight-to-strength ratio while maximizing the factor of safety of the apparatus against failure;
(b) to provide a test apparatus that reduces the weight-to-strength ratio by eliminating the application of moment loads to members of the test apparatus;
(c) to provide a test apparatus that resists failure and excessive deformation due to buckling, specifically by providing a plurality of load application members such as hydraulic cylinders;
(d) to provide a test apparatus that applies the test load to the container in a manner so as to accurately simulate the typical loading conditions applied to the container during use, specifically by applying and distributing said test loads so as to induce stresses within various members of the container; and,
(e) to provide a test apparatus that accurately measures the applied test load, specifically by reducing extraneous components of the applied and reaction loads and by minimizing deformation of members of the test apparatus.